1. Technical Field of the Invention
The present invention relates to a starting assistance device for mounting on a vehicle in which either the front wheels or the rear wheels are driving wheels which are driven by an engine and the remaining thereof (i.e., the rear wheels or the front wheels whichever the case may be) are idler wheels.
2. Description of the Related Art
A conventional starting assistance device is one which performs the starting assistance of a vehicle at the time of starting on a slippery road (such as, a snowy road or the like) by driving idler wheels with an electric motor. In this prior art, for the electric motor, it is desired to use an inexpensive DC brush motor which is used as a self-starter motor. However, if the electric motor remains coupled to the idler wheels also after the starting of the vehicle, the electric motor will be excessively rotated (or rotated at too high a rotating speed) by counter driving by the idler wheels. The durability of the brushes will therefore be impaired.
As a solution, the following structural arrangement is provided. Namely, there is provided a dog clutch which is switched on at the time of starting of the vehicle to thereby transmit the torque of the electric motor to the idler wheels. Once the vehicle has started, the driving of the electric motor is stopped and also the dog clutch is switched off.
If a DC brush motor is used as the electric motor, there will occur a frictional force by the friction of the brushes. Even if the driving of the electric motor is stopped, there will be operated on the dog clutch a torque which is equivalent to the frictional torque in the electric motor as a result of counter driving from the idler wheels. By the friction on the tooth side surfaces of the dog clutch, the dog clutch is locked to the switched-on condition (also called a switch-on locking) and therefore cannot be switched off any more. In such a case, the electric motor is excessively rotated as a result of counter driving by the idler wheels after vehicle has started, thereby resulting in the durability of the electric motor being impaired.
In view of the above-described disadvantages or drawbacks of the conventional devices, the present invention has an object of providing a starting assistance device in which the dog clutch can surely be switched off after the starting of the vehicle.